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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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21.1 Behavior of other Perl features in forked pseudo-processes

Most Perl features behave in a natural way within pseudo-processes.

$$ or $PROCESS_ID
This special variable is correctly set to the pseudo-process ID. It can be used to identify pseudo-processes within a particular session. Note that this value is subject to recycling if any pseudo-processes are launched after others have been wait()-ed on.
%ENV
Each pseudo-process maintains its own virtual environment. Modifications to %ENV affect the virtual environment, and are only visible within that pseudo-process, and in any processes (or pseudo-processes) launched from it.
chdir() and all other builtins that accept filenames
Each pseudo-process maintains its own virtual idea of the current directory. Modifications to the current directory using chdir() are only visible within that pseudo-process, and in any processes (or pseudo-processes) launched from it. All file and directory accesses from the pseudo-process will correctly map the virtual working directory to the real working directory appropriately.
wait() and waitpid()
wait() and waitpid() can be passed a pseudo-process ID returned by fork(). These calls will properly wait for the termination of the pseudo-process and return its status.
kill()
kill() can be used to terminate a pseudo-process by passing it the ID returned by fork(). This should not be used except under dire circumstances, because the operating system may not guarantee integrity of the process resources when a running thread is terminated. Note that using kill() on a pseudo-process() may typically cause memory leaks, because the thread that implements the pseudo-process does not get a chance to clean up its resources.
exec()
Calling exec() within a pseudo-process actually spawns the requested executable in a separate process and waits for it to complete before exiting with the same exit status as that process. This means that the process ID reported within the running executable will be different from what the earlier Perl fork() might have returned. Similarly, any process manipulation functions applied to the ID returned by fork() will affect the waiting pseudo-process that called exec(), not the real process it is waiting for after the exec(). When exec() is called inside a pseudo-process then DESTROY methods and END blocks will still be called after the external process returns.
exit()
exit() always exits just the executing pseudo-process, after automatically wait()-ing for any outstanding child pseudo-processes. Note that this means that the process as a whole will not exit unless all running pseudo-processes have exited. See below for some limitations with open filehandles.
Open handles to files, directories and network sockets
All open handles are dup()-ed in pseudo-processes, so that closing any handles in one process does not affect the others. See below for some limitations.
ISBN 9781906966027Perl Language Reference ManualSee the print edition